If a cycloalkane has only one substituent, it is not necessary to assign that substituent a number. If there is more than one substituent, then it is necessary to number the carbons and specify which substituent is on which carbon.

Methylcyclopentane

1,1-dimethylcyclopentane

1,2-dimethylcyclopentane

1,3-dimethylcyclopentane

The organic compound

could be named and numbered

1-cyclopropyl-5-ethyl-2-methylcyclohexane

and should be named

2-cyclopropyl-4-ethyl-1-methylcyclohexane

because it produces a lower numbered name (1+5+2=8 vs. 2+4+1=7).

In the following example, notice that the longer chain is the parent and the cycloalkane is the substituent.

Because the C-C bonds in cycles cannot rotate through 360 degrees, substituted cycloalkanes and similar compounds can exhibit diastereomerism. This is comparable to alkenes which show cis/trans (or E/Z) isomerism. The isomers can be named using cis/trans notation, or more rigorously using R-S notation.

Conformers, or conformational isomers, are different arrangements of the same molecule in space. Do not confuse them with any kind of true isomer as they are in every way the same molecule. The difference is in how the molecule is bent or twisted is space in any one instant of time.

The first molecule that is generally presented in a discussion of cycloalkane conformers is cyclohexane. It comes in several flavors; the main ones are the chair conformation and the boat conformation.

Boat conformation

Chair conformation

Note: In the above models, the straight lines represesnt single bonds, the lumps represent carbon atoms, and the open ends represent hydrogen atoms.

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The chair conformation (can you see how it looks like a chair?) is lower in energy than the boat conformation. This is because the two ends of the molecule are farther apart and avoid steric hinderance.

Hydrogen atoms in a cyclohexane can be divided into two types:

Axial, that point towards the top and bottom, and

Equitorial, that point out away from the edge of the molecule

When hydrogens are replaced with other, bulkier groups, it becomes apparent that the axial positions are less energetically favored than the equitorial positions. That means that, if given a choice, bulkier groups will tend to bond to cyclohexane in equitoral positions, as this reduces their steric hinderance and potential energy.